1. Field of the Invention
This invention relates to a black level compensation circuit for compensating a luminance signal.
2. Description of the Prior Art
A prior art black level compensation circuit performs black compensation as follows:
It expands a tone of a black portion of a luminance signal in the absence of a control signal to provide a crisp picture image. A black level detection circuit detects the blackest level for a given interval. If the detected blackest level is larger than a given value, the detection circuit produces the control signal to stop the black-level expansion. Such circuit is shown in FIG. 3.
Hereinbelow will be described such prior art black compensation circuit with reference to FIG. 3.
A luminance signal 101 is subjected to edge enhancement and amplification by a sharpness circuit 102 and a contrast circuit 103 respectively. Then, it is applied to a black level detection circuit 104 and a pedestal clamp circuit 105. The output signal from the pedestal clamp circuit 105 is applied to a black expansion circuit 106. The black level detection circuit 104 detects the blackest level for a given interval and is compared with a given reference level obtained from a pedestal level signal. If the blackest level is larger than the given reference level, the black level expansion circuit 106 does not perform the black-level expansion.
This black level detection circuit 104 will be described more specifically.
The luminance signal passing through the contrast circuit 103 (a negative signal) is fed to a differential amplifier having transistors 113 and 107. The transistors 108 and 109 controls the differential amplifier having transistors 113 and 107 to operate it for only a period that the luminance signal is received, i.e., scanning period. For a blanking period, a current source including a transistor 131 and a resistor 132 do not supply a current. When a potential of the input signal of the black level detection circuit 104 is higher than the base potential of the transistor 107, the transistor 113 turns on. Then, a current Miller circuit including a transistor 114 and a resistor 115 and another current Miller circuit including a transistor 117 and a resistor 116 allow to flow a current to charge a capacitor 112. That is, a maximum potential of the input signal of the black level detection circuit 104 is maintained at a base of the transistor 107. A resistor 118 determines a time constant for maintaining the maximum potential. A differential amplifier including transistor 135, resistors 136 and 137, and a transistor 138 compares the maximum potential of the black level with a pedestal level as the given reference level. The black expansion circuit 106 expands a tone of black portions of the luminance signal to the pedestal level when the maximum potential is lower than the pedestal level. When the maximum potential is not lower than the pedestal level, the black expansion circuit 106 stops this black level expansion. In the circuit, sets of resistors 123 and 125 and a transistor 124; a transistor 126 and a resistor 127; a resistor 128 and a transistor 134; a transistor 129 and a resistor 130; a transistor 131 and a resistor 132 comprise constant current sources respectively.
However, there is a drawback that the operation of the black compensation circuit changes with the sharpness signal.
The luminance signal inputted to the black level detection circuit 104 from the contrast circuit 103 is subjected to edge enhancement by the sharpness circuit 102 such that at an edge where the luminance signal indicating a shadow level is compensated to show more shadow. FIG. 4 shows the luminance signal subjected to edge enhancement. In FIG. 4, an original black level Vbo is changed to Vb1 in level at an edge portion by the sharpness circuit 102. As the result, the detected blackest level changes with edge enhancement. That is, black compensation changes with sharpness signal.